1. Field of the Invention
The present invention relates to a camera which is provided with a zoom lens and a lens capable of being mounted onto the zoom lens in a detachable manner to modify the focal length and magnifying power of the zoom lens.
2. Description of the Related Art
Conventionally available as one type of camera is the one provided with a zoom lens and a conversion lens which is mounted onto the zoom lens in a detachable manner to modify the focal length and power of the zoom lens.
Referring to drawings, the conventional arrangement of the zoom lens and the conversion lens which is mounted onto the zoom lens in a detachable manner to modify the focal length and magnification of the zoom lens is discussed below. FIG. 1 is a diagrammatic view showing the arrangement of a conventional camera comprising a zoom lens and a conversion lens which is mounted onto the zoom lens to modify the focal length and power of the zoom lens.
A zoom lens unit 100 comprises a first (front) lens group 101 mounted onto a lens casing as shown in FIG. 1. Disposed behind the first lens group 101 is a second lens group (hereinafter referred to as xe2x80x9ca variator lensxe2x80x9d) 102 for varying power. The variator lens 102 shares the same optical axis with the first lens group 101. To vary its power, the variator lens 102 is moved in a direction in parallel with the optical axis of its own by driving means (not shown).
An iris 103 is disposed behind the variator lens 102 to adjust light quantity. Disposed further behind the iris 103 is a third lens group 104 that is attached onto the lens casing. The third lens group 104 also shares the same optical axis with the variator lens 102.
Arranged behind the third lens group 104 is a fourth lens group (hereinafter referred to as xe2x80x9ca focus-compensation lensxe2x80x9d) 105. The focus-compensation lens 105 has a focusing function, and a compensation function, namely compensating for the displacement of the focal plane arising from power variation. The focus-compensation lens 105 shares the same optical axis with the third lens group 104. The focus-compensation lens 105 is moved in a direction in parallel with the optical axis of its own by the driving means so that the focusing and compensation are performed.
An image pickup device 106 such as a CCD is arranged behind the zoom lens unit 100, namely the focus-compensation lens 105. An imaging surface that bears an optical image of an object is provided on one side of the CCD 106 facing the focus-compensation lens 105.
A wide-angle attachment lens 121 is mounted onto the zoom lens unit 100 in a detachable manner to modify the focal length and power of the lens unit 100. When mounted onto the zoom lens unit 100, the wide-angle attachment lens 121 is positioned in front of the first lens group 101 in a manner that allows the optical axes of both are aligned. The mounting of the wide-angle attachment lens 121 causes the focal length of the zoom lens unit 100 to shift toward the wide-angle side.
Discussed below referring to drawings are a relationship between the focal length (the position of the variator lens 102) of the zoom lens unit 100 and the position of the focus-compensation lens 105 with no wide-angle attachment lens 121 mounted and a similar relationship but with the wide-angle attachment lens 121 mounted. FIGS. 2(a) and 2(b) illustrate, respectively, the relationship between the focal length (the position of the variator lens 102) of the zoom lens unit 100 and the position of the focus-compensation lens 105 with no wide-angle attachment lens 121 mounted and the same relationship but with the wide-angle attachment lens 121 mounted in FIG. 1. FIG. 3 shows the relationship between the position of the variator lens and the position of the focus-compensation lens with no wide-angle attachment lens mounted in FIG. 1. FIG. 4 shows a similar relationship.
A discussion of the relationship between the focal length (the position of the variator lens 102) of the zoom lens unit 100 and the position of the focus-compensation lens 105 with no wide-angle attachment lens mounted follows.
When the focal length of the zoom lens unit 100 is set to a predetermined value, the position of the focus-compensation lens 105 that results in an optical image on the imaging surface of the CCD 106, namely, the focused position of the focus-compensation lens 105 varies with the distance to the object as shown in FIG. 2(a). When the object distance remains constant, the focused position of the focus-compensation lens 105 varies with the focal length of the zoom lens unit 100, namely, the position of the variator lens 102. As a result, an optical image is obtained through the light which is focused by shifting the focus-compensation lens 105 according to the curve resulting from the focal length set and the object distance.
A discussion of how to follow the above-mentioned curve follows.
Referring to FIG. 3, the relationship between the focal length (the position of the variator lens 102) of the zoom lens unit 100 and the position of the focus-compensation lens 105 with no wide-angle attachment lens 121 mounted is illustrated by a first curve f1 that is a plot of a series of positions of the variator lens 102, z0, z1, z2, . . . , z6 versus a corresponding series of positions of the focus-compensation lens 105, a0, a1, a2, . . . , a6. Data, z0, z1, z2, . . . , z6 and a0, a1, a2, . . . a6, are stored in a lens control microcomputer (not shown). Equally, a second curve f2 is a plot of a series of positions of the variator lens 102, z0, z1, z2, . . . , z6 versus a corresponding series of positions of the focus-compensation lens 105, b0, b1, b2, . . . , b6. These data are also stored in the lens control microcomputer.
A third curve f3, however, is calculated from the first curve f1 and the second curve f2. The third curve f3 is a plot of a series of positions of the variator lens 102, z0, z1, z2, . . . , z6 versus a corresponding series of positions of the focus-compensation lens 105, p0, p1, p2, . . . , p6. These data are also stored in the lens control microcomputer.
p0, p1, p2, . . . , p6 are calculated by the following equation.
p(n+1)={|p(n)xe2x88x92a(n)|/|b(n)xe2x88x92a(n)|}*{|b(n+1)xe2x88x92a(n+1)|}+a(n+1)xe2x80x83xe2x80x83(1) 
Equation (1) determines the ratio of interior division of p0 on a line segment, b0-a0, when the focus-compensation lens 105 is positioned at p0. According to the ratio, p1 is plotted on a line segment, b1-a1. The speed of the focus-compensation lens 105 required to keep the zoom lens unit 100 focused is thus determined by the positional difference between p1 and p0 and the time the variator lens 102 requires to travel from z0 to z1.
Referring to FIG. 4, interpolation applied to the variator lens 102 in its one direction is discussed. In FIG. 4, the position of the variator lens 102 is arbitrarily set, and representative (cam) locus (the positions of the focus-compensation lens relative to the variator lens) is a plot of the positions of the variator lens 102, z0, z1, z2, . . . , zn and a0, a1, a2, . . . , an, and b0, b1, b2, . . . , bn relative to the object distance.
When the variator lens 102 is positioned at zx, not on any of the zooming boundaries represented by z0, z1, z2, . . . , zn within the range of travel of the variator lens 102 (namely, somewhere between zkxe2x88x921 and zk) and when the focus-compensation lens 105 is positioned at px, both ax and bx are given by the following equations.
ax=akxe2x88x92(zkxe2x88x92zx)*(akxe2x88x92akxe2x88x921)/(zkxe2x88x92zkxe2x88x921)xe2x80x83xe2x80x83(2) 
bx=bkxe2x88x92(zkxe2x88x92zk)*(bkxe2x88x92bkxe2x88x921)/(zkxe2x88x92zkxe2x88x921)xe2x80x83xe2x80x83(3) 
As understood from the above equations, ax and bx are determined by interpolating, respectively, between two sets of stored representative locus data (ak, and akxe2x88x921, and bk, and bkxe2x88x921 in FIG. 4) with respect to a fixed object distance according to the interior division ratios obtained from two sets of zooming boundaries (for example, zk and zkxe2x88x921 in FIG. 4) and the position of the variator lens 102 between zk and zkxe2x88x921.
As understood from Equation (1), both pk and pkxe2x88x921 are determined by interpolating, respectively, two sets of stored representative locus data (ak, akxe2x88x921, bk, and bkxe2x88x921 in FIG. 4) with respect to a fixed focal length according to the interior division ratio obtained from ax, px, and bx.
The travel speed of the focus-compensation lens 105 required to keep the zoom lens unit 100 focused is determined by the position difference between a focused position to be followed and the currently focused position in the zooming operation from a wide-angle side to a telephoto side and the time the variator lens 102 requires to travel from zx to zk.
The zoom lens unit 100 thus follows the curve representing the relationship between the focal length (the position of the variator lens 102) of the zoom lens unit 100 and the position of the focus-compensation lens 105 with no wide-angle attachment lens 121 mounted.
The relationship between the focal length (the position of the variator lens 102) of the zoom lens unit 100 and the position of the focus-compensation lens 105 with the wide-angle attachment lens 121 mounted is now discussed.
When the focal length of the zoom lens unit 100 is set to a predetermined value, the focused position of the focus-compensation lens 105 varies with the object distance as shown in FIG. 2(b). With the object distance kept constant, the focused position of the focus-compensation lens 105 varies with the focal length, namely with the position of the variator lens 102 (along a locus). As can be seen from FIG. 2(b), however, the curve representing the relationship, in terms of object distance as parameter, between the focal length and the position of the focus-compensation lens 105 is different from the curve representing the relationship, in terms of object distance as parameter, between the focal length and the position of the focus-compensation lens 105 with no wide-angle attachment lens 121 mounted. In FIG. 2(b), if the focal length exceeds a limit 301, the curves representing the relationship, in terms of object distance as parameter, between the focal length and the position of the focus-compensation lens 105 diverge out of the focusable focal length area. If the focal length is kept to within the limit 301, namely, the focal length is set to any value closer to the wide-angle side, focus adjustment by means of the focus-compensation lens 105 remains workable. If the focal length exceeds the limit 301, however, namely, the focal length is set to any value closer to the telephoto side, focus adjustment by means of the focus-compensation lens 105 is impossible. To cope with this problem, the variator lens 102 is moved until the focal length is set to be closer to the wide-angle side, and is fixed there. Therefore, the variator lens 102 is fixed at such a predetermined position, and zooming operation is disabled when the wide-angle attachment lens 121 is mounted.
When the variator lens 102 is set to the fixed position with the wide-angle attachment lens 121 mounted, the resulting curve representing the relationship, in terms of object distance as parameter, between the focal length (the position of the variator lens 102) and the position of the focus-compensation lens 105 differs from the curve representing the relationship, in terms of object distance as parameter, between the focal length (the position of the variator lens 102) and the position of the focus-compensation lens 105 with the wide-angle attachment lens 121 mounted. The focus adjustment operation cannot follow the movement of the variator lens 102, and keeping the zoom lens unit focused cannot be attained. As a result, the zoom lens unit suffers from a noticeably unfocused state until the variator lens 102 reaches the fixed position, and thereafter the zoom lens unit takes a substantially long time to reach a focused position from the arrival of the variator lens 102 at the fixed position.
When the wide-angle attachment lens 121 is mounted with the zoom lens unit 100 positioned closer to the telephoto side, the variator lens 102 is forced to move closer to the wide-angle side. In the course of this movement, an unfocused state takes place.
The variator lens 102 is forced to move closer to the wide-angle side when power is switched on with the wide-angle attachment lens 121 mounted onto the zoom lens unit 100. Thus, an unfocused image output and varied field of view result during the forced movement of the variator lens 102. If the mounting of the wide-angle attachment lens 121 goes unnoticed, these may be confusing to an operator.
It is a first object of the present invention to provide a camera which performs a forced zooming operation with a conversion lens group such as a wide-angle attachment lens mounted while keeping a focused state and offers an improved response characteristic in automatic focus adjustment.
To achieve the above object, according to a preferred embodiment of the present invention, a lens control device comprises a zoom lens unit having a variator lens group and a focus-compensation lens group which compensates for in a focused position a variation arising from power variation operation of the variator lens group, a conversion lens group which is mounted in an optical axis of the zoom lens unit in a detachable manner, and which restricts a focusable focal length area of the zoom lens unit to a second focal length area by shifting from a first focal length area while the conversion lens group is mounted, lens position control means for determining a focused position of the focus-compensation lens group while the variator lens group is moving, on the basis of both focused position data, predetermined according to object distance, of the focus-compensation lens group relative to the variator lens group and current position data of the focus compensation lens group and the variator lens group, and correction means for shifting the variator lens group to the second focal length area when the variator lens group is mounted in the optical axis of the zoom lens unit, and compensating for the focused position of the focus-compensation lens group in accordance with the shifting of the variator lens group to the second focal length area to follow the variator lens group.
It is a second object of the present invention to provide a lens control device and a camera, both of which avoid an operator""s confusion attributed to the fact that the mounting of a conversion lens group such as a wide-angle attachment lens goes unnoticed, and both of which offer an improved image quality and improved automatic focus adjustment performance.
To achieve the second object, according to a preferred embodiment of the present invention, a lens control device or a camera comprises a zoom lens unit having a variator lens group and a compensation lens group which compensates for in a focused position a variation arising from power variation operation of the variator lens group, a conversion lens group which is mounted in an optical axis of the zoom lens unit in a detachable manner, and which shifts a focusable focal length area of the zoom lens unit to a second focal length area from a first focal length area while the conversion lens group is mounted, position detecting means for detecting a position of the compensation lens group on the basis of a predetermined reference position, and reference position shifting means for shifting the predetermined reference position to a reference position corresponding to the second focal length area when the conversion lens group is mounted on the zoom lens unit.
It is a third object of the present invention to perform optimum control by compensating for in optical characteristics a variation resulting from the mounting of accessories.
It is a fourth object of the present invention to provide a camera which prevents a degraded quality image from being output for its initial setting, during which a variation involved in the mounting of accessories in optical characteristics is compensated for.
These and other objects and features of the present invention will be more apparent when the following detailed description is considered with the attached drawings.